Counterfeiting of goods, financial and identification documentation is a growing worldwide problem and has generated an industry in providing means to identify and authenticate genuine products. Optically variable devices (OVDs) provide a first line of defense by creating labels, hot foils, threads and laminates as both deterrent and authentication devices providing multiple level security protection and through the visual overt feature, a simple public recognition feature. Several technologies, such as holograms and color shifting inks, are used to create an image that in some way alters as the observer moves or rotates the security device. This may include changes in the image, switching from one image to another, changes in the color and several other variable effects that provide a means to identify the genuine from the counterfeit. Changes to the visual appearance of these security images provide clear differentiation from copies made by color photocopiers, for example, which cannot replicate these effects.
Technology development in the field of OVDs has included the use of liquid crystals to create an image that alters color as the angular view is changed. It is known that liquid crystals in a chiral nematic phase (cholesteric) form a helical structure that reflects circular polarized light over a narrow wavelength band. The color reflected back is created by a process referred to as constructive interference. The particular color is determined by the pitch of the helix and the angle of view. In its natural state, the individual helix elements are randomly oriented, so the incident light is scattered in all directions and no color is observed. When the material is aligned so that the cholesteric helical axis is perpendicular to the plane of the layer/substrate, for a majority of the structures, a strong color can be observed at a particular angle of view. The condition for constructive interference also occurs at a second more acute angle of view. Thus, a second, shorter wavelength spectrum is seen so that color one changes to color two as the angle of view is changed.
It is known that certain liquid crystal materials exhibit different colors when viewed in transmission or reflection and different intense colors are seen at different angles of view (U.S. Pat. Nos. 4,637,896; 4,614,619 and 4,893,906).
As competitive technologies continue to develop and counterfeiting skills and knowledge continue to increase, the ability to differentiate one security technology from another or from a counterfeit has greater importance. Increasing the range of color elements or effects achieves this differentiation and is described in the prior art. WO2005105474 describes a first optically active layer of cholesteric liquid crystal and a second partially active layer of cholesteric liquid crystal superimposed directly over the first layer. Reflected light from the two layers mix in an additive way to create a new range of color effects. WO2008046537 describes a similar method in which two layers of cholesteric liquid crystal are superimposed. The first layer is in the form of a motif or graphic design. The angular color selectivity of the layers are controlled so that the first layer reflects light into a green spectrum seen from one angle of view and the second layer reflects light into a green spectrum seen from a different angle of view.
A method of manufacturing a patterned layer of polymer material is described in WO200034808. A layer of cholesteric ordered liquid crystal material is patterned by the process of a convertible compound that when irradiated with certain energy sources alters the pitch of the helix structure in the exposed areas. The change in pitch alters the reflected and transmitted light. The material is then polymerized/cross linked by a UV source to freeze the layer and fix the perceived color. In this way a color pattern can be achieved. In EP0982605 there is described a chain terminating agent added to polymerizable material that absorbs the radiation used for polymerization and alters the pitch of the cholesteric helical structure. Variation in the radiation source used for polymerization alters the helical pitch and therefore, the reflected and transmitted colors.
WO200034808 and EP0982605 have significant drawbacks for application in manufacturing processes. For example, WO200034808 employs dose-dependent photo-isomerizing components where the absolute twisting power is low and the difference in twisting power between the two isomeric forms is modest. These factors combine to require a high concentration to achieve a visually appealing effect. EP0982605, for example, employs a combination of alignment-, temperature- and polymerization-induced phase change events to influence dose-dependent helical pitch variance that complicates manufacturing processes.
In the prior art, a material is typically applied as a single color with a graphic that is applied in the process as an area where the liquid crystal alignment is removed so that the image appears dark against the liquid crystal background color. There is a desire in the art to create an image that introduces a second color as opposed to black. There has been demonstrated that two colors may be exhibited in separate graphics, but there has not been shown exhibiting two colors in close register. It has been suggested that ink jet printing may potentially offer the ability to print several different mixes each reflecting a different color, but conventional ink jet technology is not capable of accommodating the liquid crystal material specifications, in particular, the high viscosity. Thus, there is a desire to create more than one color in register. Accordingly, there is a need in the art to develop new liquid crystal materials and methods that can be employed with ease in manufacturing processes to produce OVDs which exhibit more than one color in register. The invention relates to achieving this objective utilizing a dopant approach.